I’m having a very similar problem. I have my movie in Premiere, key our my green screen in AE, then back to premiere. When I render in premiere I get this (the video is a spoof, I don’t actually think social media is a waste of time): https://www.youtube.com/watch?v=mksLVK4818M
You say that you converted your ACVHD files into MPEG2- BluRay files. How did you do that? Then, after you did that, did you then open them in AE, work your magic, then export?